In order to fully understand changes in brain function during aging, it is necessary to understand how hormones that decline with age, such as estradiol (E) and dehydroepiandrosterone (DHEA) maintain the normal plasticity and resilience of the adult brain. Estradiol is the best studied hormone with respect to cognitive decline and Alzheimer's disease; yet, even then, we are largely ignorant about how the aging brain responds to E at the cellular and at anatomical levels, or whether E (and DHEA) have neuroprotective actions that are related to their ability to regulate plasticity. This project utilizes cell culture of embryonic hippocampal neurons that are allowed to mature for at least 10d in vitro to differentiate and establish connections among neurons with an environment of glial cells. It uses not only cell culture, with immunocytochemistry and in situ hybridization, but also employs the technique of differential display to identify novel and known genes that are activated during the induction of resilience and synapse formation. The project has a three-fold objective: First, to study the processes at the cellular and molecular level by which estradiol (E) causes synapse formation in hippocampal neurons; this will be relevant to the studies of the hypothesized age-related decline in E- induced hippocampal synapse formation in the aging rat and primate brain which are being studied in other components of this program project; Second, to study how the conditions of E treatment that results in synapse formation alter the response of the hippocampus to excitotoxic damage, and , in this connection, Third, to investigate the possible synergistic neuroprotective effects of another steroid that declines with age, dehydroepiandrosterone (DHEA). DHEA has been reported to block NMDA-induced excitotoxicity and to do so, at least in part, by acting on glial cells, which are also responsive to estrogens. Glial cells also participate in E-induced synapse formation and the information gained in these studies will illuminate this aspect as well.